1. Field of the Invention
The present invention relates to an apparatus for controlling a charge/discharge state of a hybrid car, incorporating a generator arranged to be driven by an engine and a motor for driving a vehicle, and more particularly, to an apparatus for controlling a charge/discharge state of a hybrid car that can quickly control a battery temperature to a predetermined temperature. The present invention also relates to a method for controlling a charge/discharge state of a hybrid car.
2. Description of the Related Art
A hybrid car incorporating a generator that is driven by an engine and a motor for driving a vehicle has been known. This hybrid car is designed to charge a battery with electric power obtained by the generator. Moreover, electric power discharged from the battery is used to rotate the motor so as to run the vehicle. In a usual state, a rotating force of the engine can directly be transmitted to the car's wheels. Moreover, a regenerative braking operation is performed as a braking operation corresponding to the engine brake. Also, electric power generated from the motor as a result of the regenerative braking is used to charge the battery.
Power generation in such a hybrid car is controlled so that an amount of charge (a state of charge which is hereinafter referred to as an "SOC") of the battery satisfies a predetermined range. That is, when the SOC of the battery is 0%, the vehicle is disabled to run. When the SOC is 100%, the battery cannot receive electric power generated by the regenerative braking operation. Therefore, a predetermined target SOC (for example, 50%) is set, and power generation performed by the generator is controlled so that the SOC of the battery becomes the target SOC value. As a result of the foregoing control, the SOC of the battery is allowed to always satisfy a predetermined range of about 50% (ranging from, for example, 20% to 80%).
The performance of the battery, however, deteriorates in a state where the temperature is low. When a battery that is capable of outputting electric power of about 21 KW in an optimum temperature state, such as for example, 20.degree. C. to 40.degree. C., is instead operated at 0.degree. C., the obtained output is as low as about 5 KW. In principle, the hybrid car is designed to operate the battery in a case where the output from the engine is assisted, for example, full acceleration is performed or the hybrid car is started. If the temperature of the battery is low as described above, and in particular, in case of an early morning in winter or in a cold region, a predetermined output cannot be obtained. Thus, there arises a problem that the hybrid car is prevented from performing smooth starting and acceleration. If the temperature of the battery is at a temperature not exceeding a predetermined temperature, a method may be employed with which warming-up of the engine is performed to raise the ambient temperature of the battery so as to increase the temperature of the battery to the predetermined value. In the foregoing case, however, warming-up has to be performed for a long time. Also in the foregoing case, quick start and acceleration at the required timing cannot be performed.